The present invention relates to a liquid crystal device in which a liquid crystal composition is disposed in a gap between a pair of substrates each provided with an electrode and a liquid crystal orientation control layer (particularly, a liquid crystal display device in which a pair of substrates each provided with a transparent electrode and an orientation film in that order are disposed opposite to each other leaving a predetermined gap therebetween and a ferroelectric liquid crystal composition is disposed in the gap) and a manufacturing method therefor.
Recently, studies and developments on application of ferroelectric liquid crystal (FLC; ferroelectric liquid crystal) showing bistability to display devices have been actively promoted. FLC displays are excellent display means having mainly the following features (1) to (3):
(1) High-speed response (1000 times as high as that of the conventional nematic liquid crystal display).
(2) Little visibility angle dependency.
(3) Image memory effect.
Since the ferroelectric liquid crystal display in a bistable mode has a memory effect, it can suppress flicker which is a problem in a CRT (Cathode Ray Tube) or the like, and it can be driven with scanning lines of 1000 or more even by a simple X-Y matrix driving system (driving with TFT: thin film transistors becomes unnecessary). In addition, nematic liquid crystal which is mainly used at present has a problem that its angle of visibility is small, however, the ferroelectric liquid crystal has an advantage that it has a broad angle of visibility because it has uniform molecular orientation and its panel gap is equal to or less than one half of that of a nematic liquid crystal panel.
The FLC display device (ferroelectric liquid crystal display device) 1 using such ferroelectric liquid crystal as described above has a cell structure which is schematically shown in FIGS. 1 and 2, for example. The cell structure 1 shown in FIGS. 1 and 2 mainly comprises a laminate 1A and a laminate 1b. The laminate 1A is formed by successively laminating, on the inner surface of a transparent substrate 2a of glass or the like, a transparent electrode layer 3a such as ITO (indium tin oxide: conductive oxide formed of indium doped with tin) or the like and an SiO oblique deposition layer 4a serving as a liquid crystal orientation film which has a high contrast and achieves excellent domains, and the laminate 1B which is formed by successively laminating, on the inner surface of a substrate 2a of glass or the like, a transparent electrode layer 3b and an SiO oblique deposition layer 4b. The laminates 1A and 1B are disposed so that the SiO oblique deposition layers 4a and 4b serving as the liquid crystal orientation films, and then granular spacers 5 are sandwiched between the laminates 1A and 1B to achieve a predetermined cell gap d, whereby a liquid crystal cell is fabricated. The peripheral portion of the cell is sealed with an adhesive 7 and the ferroelectric liquid crystal 8 is filled into the cell gap through the inlet port 7a and the peripheral portion of the cell is sealed with an adhesive.
For the liquid crystal orientation films 4a, 4b, there has conventionally been used an oblique deposition film of SiO, a rubbing film of polyimide or the like. Particularly, the rubbing film is practically and widely used as an orientation film for liquid crystal display devices at present. When the rubbing film is used as an orientation film, it enhances productivity and facilitates a large-area design. Therefore, at present, many liquid crystal display devices in which rubbing films are used as orientation films have been industrially produced.
However, the rubbing film causes dust to generate at the rubbing time and thus defective picture elements are liable to occur. Further, it is difficult with the rubbing film to provide a pre-tilt angle, and thus there occurs a problem that without such pre-tilt angle, picture elements become defective resulting in reducing the degree of contrast.
On the other hand, when the SiO oblique deposition film is used as the orientation film, it is difficult that pillars to determine the orientation of the liquid crystal are formed uniformly and to a large area. Further, due to the use of a vacuum deposition method, there arises the problem of bad productivity.
The SiO oblique deposition film can be manufactured by a method which is schematically shown in FIG. 3. That is, in a vacuum deposition device 35, a deposition source 37 is disposed substantially spotlike in a vacuum chamber 36, and the deposition is performed while an angle xcex81 of intersection between a line 38 connecting the deposition source 37 and the substrate 2a or 2b serving as a deposition position and a normal 39 of a deposition plane is set to about 85 degrees (the substrate 2a (2b) is disposed within xc2x1xcex8 (for example, xc2x15xc2x0) with respect to the upward and perpendicular direction of the deposition source 37).
A vacuum pump 41 is linked to the vacuum chamber 36 through a vacuum valve 40, and a predetermined vacuum degree is achieved under evacuation of the pump. Nitrogen gas is introduced into the vacuum chamber 36 through the vacuum valve 40 in a purging process. The deposition source 37 having SiO 24 mounted in a deposition boat 43 is based on a resistance heating method which applies electric current between resistance heating electrodes 42, and a deposition speed is controlled by utilizing feedback from a quartz oscillator type thickness monitor 44.
However, in the case of the above-described oblique deposition method, the precision of the angle xcex81 must be within several degrees in order to make the structure of the SiO oblique deposition film uniform (that is, in order to obtain uniform crystal orientation). The distance between the deposition source 37 and the deposition portion 2a (2b) affects the shape of the SiO pillars. Therefore, as the substrate 2a (2b) becomes large-sized, the deposition direction is diffused with a solid angle. Therefore, the angle of intersection between the deposition direction and the substrate and the distance from the deposition source are different between the edge and the center of the substrate. As a result, the structure of the oblique deposition film becomes ununiform, and the orientation characteristic does not become uniform, too.
By increasing the distance between the deposition source and the substrate as the substrate becomes large-sized, the problem of the angle or the distance between the two can be temporarily solved. However, in order to increase the distance between the deposition source and the substrate, the chamber of the deposition apparatus must be made large and also the vacuum degree must be increased, resulting in reducing the productivity to a great degree.
Therefore, it has been attempted for a long time that the orientation of the liquid crystal is controlled on the basis of the shape of the surface of the orientation film.
For example, there is a technique in which in consideration of the recognition that the liquid crystal orientation regulating force of a rubbing film is dependent on fine irregularities formed on the surface of the rubbing film or the molecular orientation of the rubbing film, the former effect is modeled and used. As described above, the orientation of the liquid crystal by providing the groove-shaped structure on the surface of the film, and its theoretical interpretation are reported in Physical Review A 24, 5, 2713-2719, (1981), Molecular Crystals and Liquid Crystals 23, 215-231, (1973), Liquid Crystals, 16,6, 1027-1036, (1994), Japanese Journal of Applied Physics 23, 2, 137-141, (1984), Journal of Applied Physics 73, 7, 3299-3304, (1993), or the like.
Besides, it has been attempted that liquid crystal orientation is controlled according to the fine shape of the film. However, a method using photoresist (Japanese Laid-open Patent Application No. Hei-3-296721, Japanese Laid-open Patent Application No. Hei-4-324426) is unsuitable for mass production because it does not adopt a replica technique such as an ultraviolet-rays hardening resin method, an injection molding method or the like.
In Japanese Laid-open Patent Application No. Hei-5-249465, it is proposed that a structure 45 having a sawtoothed section is provided on the surface of the orientation film 46 to provide a pre-tilt angle to the ferroelectric liquid crystal molecule 6 indicated by a virtual line in FIG. 4.
However, according to the knowledge of the present inventors, it is likely that with only the above-mentioned sawtoothed structure 45, liquid crystal molecules 6 are oriented along step portions 45a of the sawtooth (in a direction perpendicular to the sheet surface in FIG. 4) when an orientation film material of horizontal orientation (for example, polyvinyl alcohol) is used so that the pre-tilt angle cannot be provided. As a consequence, the contrast is not improved and the bistability which is the merit of the ferroelectric liquid crystal cannot be obtained.
Further, when a material of vertical orientation (for example, material formed of polytetrafluoroethylene) is used as the orientation film, the liquid crystal molecules 6 are oriented only vertically like the case where no sawtooth is provided if the tilt angle of the slant surface of the sawtoothed structure exceeds about 30 degrees. If the tilt angle of the slant surface of the sawtoothed structure 45 is below about 30 degrees, the liquid crystal molecules becomes inclined to some degree from the vertical orientation so that although the molecules are uniaxial oriented they have no storage property. Therefore, there is a problem that no switching characteristic is obtained, and there is a demerit when applied to the ferroelectric liquid crystal.
On the other hand, Japanese Laid-open Patent Application No. Hei-5-188377 discloses a method of providing a groove-shaped structure on the surface of an orientation film to uniaxially orient nematic liquid crystal, and at the same time, design the section parallel to the grooves in a sawtoothed structure, thereby providing a pre-tilt angle.
In this well-known technique, since the grooves are formed by using interference of laser beams (holography), a large-sized orientation film is not obtained, but it is expected that a device having an excellent orientation characteristic and a normal degree of switching characteristic can be provided when this technique is applied to a liquid crystal device using nematic liquid crystal and TFTs (thin film transistors) in combination.
However, as a result of investigation by the present inventors, it has been found out that in case where a combination of the orientation film of the well-known technique and the ferroelectric liquid crystal is used, if TFTs are used, a switching operation can be performed but if a simple matrix is used, the switching operation cannot be expected by actual application of a voltage thereto although the liquid crystal composition is oriented.
An object of the present invention is to provide a liquid crystal device which can solve the above problems, and has excellent characteristics (particularly, a ferroelectric liquid crystal device having an excellent contrast and can be made large-sized with ease), and also to provide a method which can easily manufacture such liquid crystal device in a short time and at low cost.
In order to solve the above problems, the present inventors have found out that in an orientation film which has on the surface thereof grooves and a sawtoothed structure along the grooves, the radius of curvature of the grooves is the main cause of affecting the switching operation of the ferroelectric liquid crystal, discovered an effective countermeasure, and achieved the present invention.
That is, according to the present invention, a liquid crystal device in which a pair of substrates each having an electrode and a liquid crystal orientation control layer are disposed in opposite relationship with each other leaving a predetermined gap therebetween and liquid crystal is disposed in the gap, wherein a plurality of grooves are formed on each of the inner surfaces of the substrates contacting the liquid crystal, the inner surface has a section forming repetitive asymmetrical projections in a direction along the grooves and the radius of curvature of the groove in a direction intersecting the groove is 0.1 xcexcm or more.
As a method of manufacturing the liquid crystal device of the present invention, a master (mold) for forming the grooves and the projections is preferably formed by a cutting method using laser irradiation.
The present inventors have made the following various investigations in the process of achieving the liquid crystal device and its manufacturing method of the present invention.
The present inventors formed a ferroelectric liquid crystal device having an orientation film (hereinafter referred to as an orientation film X) as disclosed in the Japanese Laid-open Patent Application No. Hei-5-188377. However, the different point from the liquid crystal devices disclosed in the Japanese Laid-open Patent Application No. Hei-5-188377 resides in that a simple matrix system is used, the gap between glass substrates is set to 1.5 xcexcm in order to apply ferroelectric liquid crystal and the ferroelectric liquid crystal is used in place of nematic liquid crystal.
A driving operation of the liquid crystal device thus formed was tried to be driven, however, it showed no switching operation even by applying a voltage of xc2x150 V. The direction of orientation of the liquid crystal was parallel to the direction of the grooves on the substrate.
Normally, in the ferroelectric liquid crystal, the orientation direction of the liquid crystal molecules 6 is switched between two states 1 and 2 in smectic phase with respect to an interaction between an externally applied electric field E and a spontaneous polarization Ps as shown in FIG. 5. Viewing the device from right above, the center axis of a circular cone which is virtually shown in FIG. 5 coincides with the direction of the orientation film (the rubbing direction for a rubbing film, the deposition direction for an SiO oblique deposition film).
However, in the liquid crystal device as described above, the orientation direction of the molecules of the ferroelectric liquid crystal itself is coincident with the direction of the orientation film (the direction of the grooves) (referred to as a state 0) as shown in FIG. 6, and neither the state 1 nor the state 2 appears. This means that the energy in the state 0 is lower than the energy in the state 1 and the energy in the state 2.
Therefore, the present inventor considered that the orientation regulating force of the grooves of the orientation film X to the liquid crystal molecules was strong, and the stabilization of the energy of the orientation regulating force in the state 0 was larger than the stabilization (states 1,2) of the energy based on the interaction between the ferroelectric liquid crystal and the electric field when the electric field was applied. On the basis of this recognition, the present inventors tried to weaken the orientation regulating force of the orientation film and perform the switching operation of the liquid crystal molecules as much as possible.
The formation of grooves on an orientation film as described above has hitherto been performed by using the interference of laser beams (holography). This method has had a problem that an orientation film having a large-area pattern cannot be formed because an area where a holograph is obtained is narrow. In addition, from the point of view of control of the orientation regulating force of the orientation film, there is a problem that the controllability of the shape is low. When the holographic technique is used, the width and depth of the grooves are parameters which can be controlled. The sawtoothed structure must be formed by another method.
Therefore, the applicant has considered another method of forming the shape of the orientation film by scanning laser beams while modulating the laser beams by AOM (Acoustic-Optic Modulator). With this method, the grooves and the sawtoothed section can be formed at the same time by varying a modulation pattern of the AOM, the xcex3-characteristic of the resist being used, the wavelength and focus of the laser, etc., and in addition, the pitch and angle of the sawtoothed structure, the width and depth of the grooves, and the sectional shape (the radius of curvature of projections and recess portions of the grooves) can be controlled.
As a result of investigation of the shape of the orientation film as described above, the present inventors have found out that the sectional shape of the grooves, particularly the minimum radius of curvature of the recess portions of the grooves in a direction perpendicular to the longitudinal direction of the grooves gives an adverse effect on the switching operation of the ferroelectric liquid crystal. That is, it has been found out that the minimum radius of curvature of the recess portions of the grooves of the orientation film formed by the holography is so small that the switching operation of the liquid crystal molecules is adversely affected.
At the same time, it has been found out that there is sometimes a case where when the minimum radius of curvature of the recess portions of the grooves is so set as to allow the switching operation to be performed, the liquid molecules are not oriented in the direction of the grooves, but oriented along step portions formed by the sawtoothed structure (in a direction perpendicular to the grooves).
Therefore, the present inventors have investigated the balance between the orientation force due to asymmetrical projections like the sawtoothed structure and the orientation force due to the grooves, and sought a condition required for the liquid crystal molecules to be oriented in the direction of the grooves while the liquid crystal molecules keep pre-tilt. As this condition, the grooves is indispensably formed so that the recess portions of the grooves in a direction intersecting (particularly in a direction perpendicular to) the longitudinal direction of the grooves has the minimum radius of curvature of 0.1 xcexcm or more.
That is, since the orientation regulating force of the liquid crystal molecules by the grooves can be controlled by the radius of curvature in the specific range as described above, the switching operation of the liquid crystal molecules is facilitated, and the liquid crystal molecules are not oriented along the steps of the asymmetrical projections like the sawtoothed structure. In addition, the liquid crystal molecules can be oriented at a desired pre-tilt angle along the grooves by the projections as described above. Particularly for the ferroelectric liquid crystal, the switching operation can be easily performed at a desired cone angle on the surface of the circular cone shown in FIG. 5.